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LM43603: Phasemargin and Webbench

Part Number: LM43603

I'm having problems with a design using the LM43603. I'm using several of them with different output voltage. But for this case I will just mention specs: Vout 3.3V, 1.0 A. Vin 10-33V

I'm using the recommended approach in the datasheet. and default switch frequency of 500 kHz The inductor is selected to reduce ripple current (datasheet recommend 20-40%). The selected inductor is 39 uH since I aim for a lower rpple current. However, the webbench suggest a much smaller inductor (5.2 uH). Clearly the ripple current exceeds datasheet max recommendations.
In webbench I have tried customizing the design and select L=39 uH. I also select Rfbt=220k and Rfbb=97.6k and adjust Cff accordingly. However, I'm unable to freely select Cout. It is limited to around 47 uF, but I have more. So in webbench I cannot simulate with the actual Cout and the value of Cff calculated using the equation in the datasheet.
Anyway, with the possible design changes, webbench claims the design i unstable. Phase margin becomes too small for low Vin.

Unfortunately I cannot simulate my exact design due to the limitations of webbench and the datasheet does not provide sufficient information to calculate phase margin myself.

Questions:

How is phasemargin calculated for the LM43603? What is the loop transfer function?

Is there a work-around to allow free selection of output capacitors for the Webbench?

  • nphansen,

    Yes you can select the output capacitor in WEBENCH by clicking the capacitor, selecting "CHOOSE ALTERNATE" and editing the search conditions at the top of the Alternate Part Selection window.

    Also 39uH will give between 12% and 16% ripple for 10-33VIN, 3.3VOUT, 1A. I suggest something more like 18uH.

    -Sam